The invention relates to a method for forming a state signal to indicate a bypass operation in a high voltage DC transmission system. More particularly, in a system including a converter, coupled to ends of a high voltage DC transmission line ("HVDC" line) and acting as a rectifier or inverter, after a normal commutation sequence is interrupted, the state signal indicates a successful commutation of the DC current to selected converter valves The successful commutation makes possible the flow of DC current and causes a bypassing of a side of a converter transformer connected to the converter. The bypassing of the side of a converter transformer is a "bypass" operation. Further described is a particularly advantageous apparatus for carrying out the method according to the invention.
High-voltage DC transmission lines ("HVDC" lines) may be terminated at both ends by converters which in turn are coupled, via converter transformers, to an AC network which, as a rule, has several phases. One of the two converters works here as an inverter and the other one works as a rectifier. If a multi-pole fault occurs in one of the two AC networks connected to each other by the converter transformers, converters and the HVDC line, it may be advantageous to initiate a so-called "bypass" operation in one of the two converters. In the case of an AC network disturbed by a two or three-pole short circuit, the affected network is separated from the one side of the HVDC line by the bypass operation. The flow of DC current on the HVDC line can thereby still be maintained for a certain amount of time with reduced nominal value. During a bypass operation no energy is transmitted between the AC networks. The bypass is particularly advantageously in avoiding a fast switching-off of the entire HVDC line in cases of a network fault of only short duration and it avoids a "start-up procedure" which is then necessary after correcting such a fault. The switching-off and start-up procedure are time consuming and may cause voltage variations in the adjacent AC networks
A temporary initiation of a bypass in one of the converters can also be used when network filters are connected in one of the AC networks during a "start up procedure" of the HVDC line. The brief decoupling of the two AC networks, normally connected via the HVDC line, by the bypass, makes possible a time-coordinated connection of network filters and a fast control of the reactive power variations in the respective network that normally occur with such a connection. Excessively large transient network variations are also avoided using the bypass operation. After the decay of the transients caused by the addition of the network filters, the converter in question is s switched back to normal operation
For initiating a bypass in one of the converters coupled to the HVDC line, a normal commutation sequence of valves of the converter is first interrupted. However, the valves, the current flow of which is now no longer relieved, still permit a flow of DC current through the side of the converter transformer connected to that converter. This current should be interrupted as fast as possible in order to avoid phenomena, such as premagnetization, from occurring to an increased degree in the magnetic circuit of the transformer. Such phenomena may impede fast switching of the HVDC line back to normal operation. To this end, the DC current must be commutated away from one of the valves still carrying current. This is accomplished by firing at least one selected converter valve. A DC current flow that bypasses the side of the converter transformer connected to the converter is made possible in this manner.
The occurrence of a multi-pole fault in one of the adjacent AC networks is recognized by a network monitor after about five to ten msec. A bypass, requested thereafter, is initiated by the successful commutation of the DC current to the selected converter valve or valves approximately twenty msec after the detection of the fault. In practice however, there is the danger that the necessary DC commutation is not successful. This can be caused by disturbed line voltages coupled to the converter via the converter transformer which can be loaded, for instance, with voltage increases, high frequency AC components or DC terms. Thus, the initiation of a requested bypass operation must be monitored for success or failure by the formation of a corresponding state signal so that, if required, other protective measures, such as a fast shutdown, can be initiated in time.
The disappearanoe of a current flowing between the respective converter and the corresponding converter transformer as the result of a successful initiation of a bypass has been monitored heretofore in a rather complicated manner. One such method uses "intermediate transformers.-+. which are additionally connected in HVDC systems for measuring the current flowing to the existing AC main transformers on the side of the converter transformer connected to the converter. To this end, the secondary windings of the AC main transformers are short-circuited and supply, as a rule, several further transformers through whose primary side the main transformer secondary current flows. The secondary windings are connected in series. These are terminated with a load resistor and are designed separately in the magnetic circuit in accordance with a specially desired measuring application, for instance, for generating control signals and signals for addressing protective devices The intermediate transformers belong to this group and are AC current transformers designed for measuring the current as linearly as possible, even in the overcurrent range. The sum of the absolute amounts of the currents measured in this manner is utilized for monitoring the bypass.
Each of the main and intermediate transformers must be designed here so that the network currents, which are coupled over from the network side of the respective converter transformer and are as a rule superimposed by transient DC components, are imaged accurately enough at least for a certain period of time. This period of time should be at least as long as is required for the successful initiation of a bypass operation in the normal case. AC transformers of this type, which can transmit shifted short-circuit AC currents with superimposed transient DC components, in addition to a steady-state AC short-circuit current, to an ohmic load with the full information content, must be heavily overdesigned in the transformer core and the windings. This leads to large and costly designs and can lead to rather high load powers. The nominal overcurrent figure serving for the design of such AC converters must be determined taking into account the time constant of the connected AC network. This network time constant, however, is not always available and furthermore is a function of the current variable load state of the network.